The present invention relates to a lithium secondary battery for use in electric vehicle. More particularly, the present invention relates to a lithium secondary battery suitably used as a battery for driving the motor of an electric vehicle (including a hybrid electric vehicle).
In recent years, as the movement for environmental protection is increasingly becoming active, it has been investigated in the automobile industry to introduce, at a timing as early as possible, electric vehicles (EVs) or hybrid electric vehicles (HEVs) in place of conventional gasoline vehicles and the like using fossil fuels, and development of a battery used for driving of motor has become active because the battery is a key for practical use of electric car.
As the battery for EV or HEV, a lithium secondary battery has drawn attention in recent years for the high energy density. This battery can therefore give a long running distance covered per one charging operation, as compared with conventional lead-acid battery or metal-hydride battery.
The lithium secondary battery uses a lithium compound as the active material of positive electrode and a carbon material as the negative electrode. In the battery, the lithium ion in the positive electrode active material moves into the negative electrode active material during charging and, during discharging, the lithium ion captured by the negative electrode moves into the positive electrode; thereby, charging and discharging is conducted.
As one preferred structure of the lithium secondary battery, a structure having an electricity-generating body of wound-type is proposed. This wound-type battery is produced by placing, in a cylindrical battery case, an electricity-generating body 1 as shown in FIG. 2, obtained by winding a positive electrode 2 and a negative electrode 3 via a separator 4, and is suitable as a compact battery having electrodes of large area. Further in the wound-type battery the numbers of lead wires 5 leading to electrodes 2 and 3 may be each at least one and, when the resistances for collecting electricity from electrodes 2 and 3 are desired to be made small, the number of lead wires can be increased; therefore, the inside structure of battery is not complex and the assembling of battery is easy.
The structure of the lithium secondary battery also includes a lamination type. The lamination type battery has an electricity-generating body obtained by alternately laminating a positive electrode and a negative electrode via a separator. Since this structure can be designed in any shape of a rectangular parallelepiped, a disc and a cylinder depending upon the shape of each electrode and the number of lamination, the lamination type battery is suitably used when there is a restriction as to the shape of the battery used. The lamination type battery, however, has a complicated internal structure for reasons such as the necessity of lead wire for each positive electrode and each negative electrode. Therefore, the wound-type battery is superior in view of the assembling operation.
The lithium secondary battery having such a structure has a terminal voltage of about 4 V and therefore is unable to use an aqueous electrolyte solution and must use a non-aqueous organic electrolyte solution having a lower lithium ion conductivity than the aqueous electrolyte solution does. This tends to allow the battery to have a large internal resistance. Since the acceleration of electric vehicle is determined mainly by the internal resistance and output of the battery used in the vehicle, it is important to make small the internal resistance of the battery and stabilize its output.
With respect to the properties of the lithium secondary battery for use in electric vehicles, the properties of a lithium secondary battery using LiC0O2 as the positive electrode active material and hard carbon as the negative electrode active material are described in, for example, 1997 JSAE Spring Convention Proceedings No. 971 (JSAE stands for Society of Automotive Engineers of Japan, Inc.).
In designing a lithium secondary battery for use in electric vehicles, there is a generally known concept of making small the internal resistance of the battery and stabilizing its output in view of the acceleration of the vehicle, as mentioned above. However, there is no clear idea yet on how the above concept is achieved using parameters, for example, the output, internal resistance, voltage drop and weight of the battery.
In a battery for electric vehicles, it is clear that an output of a given level or higher is necessary in view of the acceleration of the vehicle. When the battery is allowed to have such a large volume that can produce an output necessary for giving sufficient acceleration, however, the volume of the battery reduces the space utility of the vehicle; the total vehicle weight becomes large and the efficiency of the battery becomes low; and an increase in battery cost raises the vehicle cost by that much.
To alleviate the above problems, the present inventors made a study; as a result, the present inventors found out that in designing a lithium secondary battery for use in electric vehicle, a ratio of battery output and battery energy is very important and further that by allowing the ratio to fall in a given range, a lithium secondary battery having properties necessary for use in electric vehicle can be produced. The present invention has been completed based on the above finding.
According to the present invention, there is provided a lithium secondary battery for use in electric vehicle, comprising:
a battery case, and
an electricity-generating body including a positive electrode, a negative electrode, and a separator, the positive and the negative electrode being wound or laminated via the separator so that the positive electrode and negative electrode are not brought into direct contact with each other,
wherein each single battery has a ratio (X/E) of battery output X (W) and battery energy E (Wh), of 2 to 36, and the lithium secondary battery is used in an electric vehicle as combined batteries formed by connecting a required number of the single batteries in series.
According to the present invention, there is also provided a lithium secondary battery for use in electric vehicle, comprising:
a battery case, and
an electricity-generating body including a positive electrode, a negative electrode, and a separator, the positive and the negative electrode being wound or laminated via the separator so that the positive electrode and negative electrode are not brought into direct contact with each other,
wherein each single battery has a product (Rxc3x97E) of battery internal resistance R (mxcexa9) and battery energy E (Wh), of 50 to 900 (mxcexa9xc2x7Wh), and the lithium secondary battery is used in an electric vehicle as combined batteries formed by connecting a required number of single batteries in series.
In the present invention, the combined batteries have an output of preferably 20 kW or more, more preferably 50 kW or more. Further, the combined batteries have a weight of preferably 100 kg or less, more preferably 50 kg or less.
Also in the present invention, the positive electrode active material is preferably lithium manganese oxide (LiMn2O4) having a spinel structure, more preferably Li-rich lithium manganese oxide having a spinel structure. The Li-rich lithium manganese oxide refers to a material composed of Li, Mn and O, wherein the proportion of Li is more (Li/Mn greater than 0.5) than in the LiMn2O4 (stoichiometric composition, Li/Mn=0.5); and includes all the materials obtained by substituting the Mn of LiMn2O4 with Li and, depending upon the case, other element(s) (e.g. Cr, Ni, Co and/or Ti).
In the lithium secondary battery for use in electric vehicles according to the present invention, the electricity-generating body is preferably a wound-type.